Methane Catalytic Combustion under Lean Conditions over Pristine and Ir-Loaded La1−xSrxMnO3 Perovskites: Efficiency, Hysteresis, and Time-on-Stream and Thermal Aging Stabilities

The increasing use of natural gas as an efficient, reliable, affordable, and cleaner energy source, compared with other fossil fuels, has brought the catalytic CH4 complete oxidation reaction into the spotlight as a simple and economic way to control the amount of unconverted methane escaping into the atmosphere. CH4 emissions are a major contributor to the ‘greenhouse effect’, and therefore, they need to be effectively reduced. Catalytic CH4 oxidation is a promising method that can be used for this purpose. Detailed studies of the activity, oxidative thermal aging, and the time-on-stream (TOS) stability of pristine La1−xSrxMnO3 perovskites (LSXM; X = % substitution of La with Sr = 0, 30, 50 and 70%) and iridium-loaded Ir/La1−xSrxMnO3 (Ir/LSXM) perovskite catalysts were conducted in a temperature range of 400–970 °C to achieve complete methane oxidation under excess oxygen (lean) conditions. The effect of X on the properties of the perovskites, and thus, their catalytic performance during heating/cooling cycles, was studied using samples that were subjected to various pretreatment conditions in order to gain an in-depth understanding of the structure–activity/stability correlations. Large (up to ca. 300 °C in terms of T50) inverted volcano-type differences in catalytic activity were found as a function of X, with the most active catalysts being those where X = 0%, and the least active were those where X = 50%. Inverse hysteresis phenomena (steady-state rate multiplicities) were revealed in heating/cooling cycles under reaction conditions, the occurrence of which was found to depend strongly on the employed catalyst pre-treatment (pre-reduction or pre-oxidation), while their shape and the loop amplitude were found to depend on X and the presence of Ir. All findings were consistently interpreted, which involved a two-term mechanistic model that utilized the synergy of Eley–Rideal and Mars–van Krevelen kinetics.


Introduction
The replacement of traditional fossil fuels with cleaner and/or sustainable energy sources is currently imperative due to the recent global energy and environmental crisis. Regarding the global transition toward so-called "low carbon footprint energy technologies" and successful sustainability, humanity has exhibited an ever-increasing dependence upon natural gas (NG); gas is widely considered to be the 'bridge fuel' during this transition period [1][2][3][4]. NG typically contains~85-95% methane [4][5][6]. The high efficiency of methane as nanostructure of the perovskite itself, thus improving the surface area and other catalysisrelated properties [54,55], new horizons were opened for the use of perovskites in various environmental- [54,[56][57][58][59] and energy- [55,[60][61][62][63][64] related catalytic processes, which resulted in great success.
However, the classical partial substitution of the A and/or B sites of an ABO 3 perovskite with other cations, with the same or different valences, to obtain substituted (as called) A 1−y A'yB 1−x B' x O 3±δ perovskites, remains a popular methodology for controlling the performance of perovskites due to its high flexibility. The bulk, surface, and redox properties of the original ABO 3 perovskite can be easily tailored on demand [43][44][45][46][47]. Emphasis is always placed on the oxygen storage capacity (OSC), oxygen ion mobility, and population of the surface oxygen vacancies (O defects) of the substituted perovskites; these properties play a key role in catalysis via oxides [56][57][58][59][60][61][62][63][64][65][66][67], as well as in catalysis via dispersed metal nanoparticles on oxide supports [13][14][15][68][69][70][71][72][73][74][75]. Regarding the latter, these properties are critical for the development of desirable metal-support interactions. It is therefore obvious why perovskites are a class of materials that attract high and ever-increasing interest with regard to heterogeneous catalysis; they are either used as pristine materials, exploiting their own advantageous characteristics for catalysis, or as effective supports for metal nanoparticles, as they endow them with favorable metal-support interactions.
In a previous study, the effect of the degree of the substitution of La by Sr on La 1−x Sr x MnO 3 perovskite and their counterpart, iridium-loaded catalysts (Ir/La 1−x Sr x MnO 3 ), was studied in CO oxidation with excess O 2 reaction [30]. Both catalyst series were found to be effective in the reaction, with the Ir/La 1−x Sr x MnO 3 catalysts significantly outperforming their pristine La 1−x Sr x MnO 3 counterparts. Interesting inverse hysteresis phenomena were observed during heating/cooling cycles, depending on both the degree of substitutional Sr (x), and the pretreatment (pre-oxidation/pre-reduction) of the catalysts. The results were part of a project to decipher the effectiveness of perovskite-based catalysts, in combination with the relatively cheap noble metal, Ir, in reactions related to the control of NG-powered vehicle emissions. Traffic caused by NG-fueled vehicles is increasing rapidly (ca. 23 million worldwide, ranging from heavy-duty to light-duty cars, with an annual increase of 20%) [4]. Catalysis of CH 4 combustion is also of particular interest because unburnt CH 4 also exists in the exhaust stream of NG-powered engines and processes [21][22][23][24][25][26][27][28][64][65][66][67][68]. Complete methane combustion is also a useful side process in the very demanding application of natural-gas (NG) fueled gas-turbines (GT), as they operate alongside the catalytically stabilized hybrid combustion concept [76].
In the present work, lean CH 4 combustion on pristine La 1−x Sr x MnO 3 (LS X M; X = 0, 30, 50 and 70% substitution of La with Sr) perovskites and their 2 wt% iridium loaded counterparts (Ir/LS X M) is comparatively studied in a temperature range of 400-970 • C. The impact of the degree of substitution of the A-site (La) of the perovskite with Sr, on its catalytic behavior is explored. This occurs after an exploration into the catalysts' various pre-treatment protocols, such as pre-reduction, pre-oxidation, and thermal aging, which were implemented to obtain a complete overview of their CH 4 combustion performance and how it correlates with the morphological characteristics and properties of the materials. Complex hysteresis phenomena recorded for the first time using this catalyst/reaction system during heating/cooling cycles were therefore consistently interpreted. Although LSM perovskites are possibly among the most studied materials for their catalytic performance in various reactions, not excluding some studies concerning the oxidation of CH 4 , the highly systematic activity and stability studies performed herein revealed new phenomena and findings that could be both of specific and general interest for catalysis research.

LSxM and Ir/LSxM Catalysts Synthesis
The unloaded and 2 wt% Ir loaded perovskite-type La 1−x Sr x MnO 3 (denoted hereafter as LSxM, where x = 0, 30, 50 and 70 expresses the % replacement of La with Sr in the perovskite formula) catalysts were prepared using the co-precipitation method described by Haron et al. [77]. The nitrate salts, La(NO 3 ) 3 ·6H 2 O (VWR Chemicals, 99.9%), Sr(NO 3 ) 2 (Sigma Aldrich, St. Louis, MO, USA, 99.0%) and Mn(NO 3 ) 2 ·6H 2 O (Panreac, Darmstadt, Germany, 98.0%), were used as metal precursors. In brief, appropriate amounts of the nitrate salts were diluted in distilled water and added to a NaOH (VWR Chemicals, Radnor, PA, USA, 98.9%) precipitating agent solution for co-precipitation. The obtained suspension was filtered, washed, dried, de-agglomerated, and finally calcined in air at 1000 • C for 6 h for the final perovskite structure to be obtained. A series of LS 00 M, LS 30 M, LS 50 M and LS 70 M perovskites were produced.
Half of each perovskite was then impregnated under continuous stirring conditions at 75 • C in an appropriate amount of aqueous solution comprising IrCl 3 ·H 2 O (Abcr GmbH & Co KG) with 2 mg Ir/mL in order to achieve the corresponding 2 wt% Ir/LS X M catalyst series. After the water evaporated, the obtained suspensions were dried at 110 • C for 12 h; then, they were subjected to reduction at 400 • C for 3 h in a 50 mL/min flow of 25% H 2 /He to remove residual chlorine and to avoid the formation of large Ir crystallites [32,33]. The as-prepared LS X M and Ir/LS X M catalyst series are listed in Table 1.

Catalyst Characterization Methods
The textural and structural characteristics and redox properties of the LS X M and Ir/LS X M catalysts were determined using the N 2 physical adsorption BET-BJH method, powder X-ray diffraction (XRD), isothermal pulse H 2 -chemisorption (H 2 -Chem.), and Temperature programmed reduction using hydrogen (H 2 -TPR).
More specifically, the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) model were used to analyze N 2 adsorption-desorption isotherms obtained at −196 • C, and relative pressures in the range of 0.05-0.3, using a Quantachrome Nova 2200 e instrument. Subsequently, 150 mg of the sample placed in the instrument holder was degassed for 12 h using a vacuum at 350 • C, prior to measurements. The total specific surface area (S BET ), the pore volume, and the average pore size diameter of the materials were obtained.
The powder X-ray diffraction (PXRD) patterns of pre-oxidized samples (calcinated in the air at 400 • C for 1 h before the XRD measurements) were collected with a BrukerAXS D8 Advance diffractometer operating at 35 kV and 35 mA, using Cu Kα radiation and a LynxEye detector with a Ni-filter, to determine the crystalline structure of the materials. Measurements were carried out in a 2θ angle range of 4-70 degrees, with a scanning speed of 0.5 degrees per min. The identification and quantification of the phases were performed using BrukerAXS Topas software (COD, Crystallography Open Database) and the Rietveld method.
Pulse hydrogen chemisorption (H 2 -chemisorption) experiments were performed at 25 • C on a Quantachrome/ChemBet Pulsar TPR/TPD instrument equipped with an Omnistar/Pfeiffer Vacuum mass spectrometer to determine iridium dispersion and mean iridium particle size. An amount of 150 mg of the material was loaded on the instrument Nanomaterials 2023, 13, 2271 5 of 26 holder, which was pretreated with a 5% H 2 /He mixture (15 mL/min) at 550 • C for 1 h. Then, it was flushed with N 2 (15 mL/min) at the same temperature for 30 min, and cooled to 25 • C using a N 2 flow, before consecutive pulse injections of pure hydrogen (280 µL H 2 per pulse) were imposed until saturation was reached. This was carried out in order to measure the total H 2 -uptake (V chem ). The V chem values were then used to calculate iridium dispersion, D Ir (dimensionless, H/Ir ratio), and mean Ir crystallite size, d Ir (in nm) via the following set of Equations (1) and (2) [15,30]: where, V Chem (mL/g) is the H 2 -uptake in the chemisorption experiment, F s is the hydrogen to metal correlation factor (=2 assuming the one-to-one correlation of adsorbed H atoms with metal sites (i.e., H-Ir)), A Ir is the atomic weight of iridium (192.22 g/mol), V mol is the molar volume of an ideal gas at room temperature and 1 atm pressure (ca. 24,450 mL/mol), X Ir is the iridium content of the catalyst (g Ir /g cat ), ρ Ir is the Ir metal density (22.5 g/mL), α Ir is the cross-sectioned area of the Ir atom (0.12 nm 2 /atom), N AV = 6.023 × 10 23 molecules/mol is the Avogadro number, and 10 20 is a unit conversion factor when the units of the parameters in Equations (1) and (2) are used as indicated above. Temperature-programmed reduction (H 2 -TPR) was performed using the same instrumentation for H 2 -chemisorption experiments in order to obtain the reducibility characteristics and determine the total oxygen storage capacity (t-OSC) of the materials. The samples (150 mg) were oxidized in situ at 750 • C for 30 min (20% O 2 /He flow), then, they were cooled to 25 • C using the same flow, purged for 10 min with He flow. Next, the TPR experiment was performed with a linear (10 • C/min) increase in temperature, up to 750 • C, using 15 mL/min of 1% H 2 /He flow. The time integral of the H 2 -TPR profile determines the total oxygen storage capacity (t-OSC) of the material [13,15,30].

Catalytic Activity and Stability Evaluation Experiments
The catalytic activity and thermal stability experiments were performed in a continuous flow experimental apparatus ( Figure S1) consisting of the following: (i) a feed unit utilizing MKS-247 mass flow meters, (ii) a reactor unit with a tubular fixed-bed type reactor (quartz, ID = 3 mm), and (iii) an analysis unit equipped with online gas chromatography (SHIMADZU GC-14B, a thermal conductivity detector, He carrier gas, Porapak-N, and Molecular Sieve 5A columns connected in parallel). The reactor was loaded with m cat = 50 mg of a catalyst in the form of a powder (particle size 180-250 µm). A Kthermocouple, centered in the catalyst bed, was used to measure the reaction temperature. The volume of the catalytic bed was ca. 0.03 cm 3 .
Catalysts were comparatively evaluated for the CH 4 combustion reaction under conditions of excess O 2 (1% CH 4 + 5% O 2 , balance He at 1 bar) at a weight-basis Gas Hourly Space Velocity (WGHSV) equal to 90,000 mL/g·h (total flowrate F T = 75 mL/min), in the temperature range of 400-970 • C; this corresponds with the operating temperature window of vehicle emission control systems (in particular, of close-coupled catalytic converters). The residence time of the reactants is ca. τ = 0.023 s (i.e., a gas hourly space velocity, GHSV, of ca. 156,500 h −1 ). The 'light-off' behavior of the catalysts (CH 4 conversion versus temperature in constant reactor feed conditions) was obtained by increasing the temperature stepwise (~30 • C/step); in other words, they remained at each temperature for~20 min to ensure a steady-state operation.
Both the pre-reduced and pre-oxidized states of the catalyst were evaluated by respectively applying the following pretreatment conditions, prior to light-off measurements: (i) pre-reduction at 600 • C for 2 h with a 50 mL/min flow of 25% H 2 /He, and (ii) preoxidation at 400 • C for 1 h with a 50 mL/min flow of 20% O 2 /He. In addition, 12 h of 6 of 26 time-on-stream (TOS) stability experiments were performed at a constant temperature, which was different for each catalyst, and equal to that of its corresponding T 50 (temperature for 50% CH 4 conversion). Finally, resistance against the deactivation of the LS X M and Ir/LS X M counterpart catalysts, after the imposition of stressed oxidative thermal aging conditions, was studied by imposing 5 h in situ oxidation at 750 • C with a 20% O 2 /He flow; then, a re-evaluation of catalyst performances under the same reaction conditions that were previously applied took place. All the above steps, and the order in which they were performed, are shown in Scheme 1.
temperature stepwise (~30 °C/step); in other words, they remained at each temperature for ~20 min to ensure a steady-state operation.
Both the pre-reduced and pre-oxidized states of the catalyst were evaluated by respectively applying the following pretreatment conditions, prior to light-off measurements: (i) pre-reduction at 600 °C for 2 h with a 50 mL/min flow of 25% H2/He, and (ii) pre-oxidation at 400 °C for 1 h with a 50 mL/min flow of 20% O2/He. In addition, 12 h of time-on-stream (TOS) stability experiments were performed at a constant temperature, which was different for each catalyst, and equal to that of its corresponding T50 (temperature for 50% CH4 conversion). Finally, resistance against the deactivation of the LSΧM and Ir/LSΧM counterpart catalysts, after the imposition of stressed oxidative thermal aging conditions, was studied by imposing 5 h in situ oxidation at 750 °C with a 20% O2/He flow; then, a re-evaluation of catalyst performances under the same reaction conditions that were previously applied took place. All the above steps, and the order in which they were performed, are shown in Scheme 1. The conversion of CH 4 (X CH4 ) is calculated using Equation (3), as follows: where F is the total gas flow rate (mL/min), [CH 4 ] is the v/v fraction of CH 4 in the gas stream, and the subscripts "in" and "out" indicate the reactor inlet and outlet gas streams, respectively.

Catalyst Characteristics and Physicochemical Properties
A thorough characterization of the LS X M and Ir/LS X M materials has been previously performed and can be found in [30]. The textural, morphological, and total oxygen storage capacity (t-OSC) characteristics are summarized in Table 1 and described in detail below.
The specific surface area (S BET ) values of the materials were between 6.8-12 m 2 /g, which are quite low, but typical for perovskite-type materials. The substitution of La with Sr, up to when X = 50%, decreased S BET monotonically from 12 to 6.8 m 2 /g, which then increased again for when X = 70%, as it approached the initial value. Perovskites are considered to be ionic materials consisting of closed-packed arrays of relatively large oxygen ions. Under these conditions the rate controlling step for densification during sintering comprises the diffusion of the oxygen anions; this takes place through oxygen vacancies rather than through interstitial sites. The addition of the substituted Sr, was compensated with Mn 4+ cations, the reduction of which, upon heating at 1000 • C, creates oxygen vacancies. These enhance the oxygen anion diffusion rate, and consequently, the densification rate. This explains the gradual S BET reduction, along with the increasing Sr content. The subsequent increase in S BET , at higher Sr contents, indicates that at Sr contents higher than 0.5, the defect chemistry of the system changes and is not dictated by the same rules. In addition, the presence of secondary phases, even at small amounts, is able to significantly influence densification during sintering at 1000 • C.
The addition of Ir via impregnation reduced the value of S BET due to the partial blocking of the LS X M small-sized pores, caused by the Ir nanoparticles. This is evidenced by the slightly higher average pore size values of Ir/LS X M compared with their LS X M counterparts ( Table 1).
The average size of Ir nanoparticles was estimated to be in the order of 1.0-1.2 nm (Table 1) using Equations (1) and (2) fitted with the H 2 -uptake values obtained from the isothermal H 2 -chemisorption experiments. The corresponding Ir dispersion values (D Ir ) ranged between 61% and 73%.
The structural characteristics of the materials obtained via the analysis of their XRD patterns confirmed the development of the lanthanum manganate perovskite structure at an angle of 2θ~32.4-33.1 • ( Figure S2 in the Supplementary Materials file). Other secondary phases of single or mixed oxides, appearing mainly in the material with the higher substitutions of La with Sr (X = 70%), can be seen in Figure S2. The main peak of the perovskite structure, at about 33 • , is magnified in Figure S3, wherein a gradual transition from a rhombohedral to a cubic structure (with increasing Sr content) is clearly distinguishable from the splitting of the peak at a diffraction angle 2θ~32-33 • . In addition, there is a shift to larger angles as the substitution of La with Sr increases; this indicates that the unit cell is being contracted [47,48]. This cell contraction most likely resulted from the oxidation of Mn 3+ to Mn 4+ when the Sr content of the material increased, rather than from the creation of oxygen vacancies, which, if they occurred, would lead to unit cell expansion [47,48]. Moreover, the appearance of other crystalline phases of single or mixed oxides, along with the perovskite structure, appeared in materials with higher substitutions of La with Sr (x = 70%). On the other hand, no Ir species were detected in the XRD diffractograms of Ir/LS X M catalysts, which is most likely due to its small-sized nanoparticles (~1 nm); this indicates successful dispersion.
The reducibility characteristics of the LS X M perovskites are shown in the H 2 -TPR profiles of Figure 1. The total oxygen storage capacity (t-OSC) of each perovskite, obtained from the integrated area of their respective TPR profiles in the time interval of the experiment [13,30], are given in Table 1. Values range from 670 to 1350 µmol O 2 /g, increasing systematically with increasing levels of X.  [13,30], are given in Table 1. Values range from 670 to 1350 µmol O2/g, increasing systematically with increasing levels of X. Figure 1. H2-TPR profiles for the LSXM catalysts and main peak deconvolutions. Peak (α) → Oads; peak (β) → Mn 4+ to Mn 3+ reduction; peak (γ) → Mn 3+ to Mn 2+ reduction.
The deconvolution of the broad overlapping peaks in the TPR profiles of LS X M reveals three main peaks (α, β, and γ) for the perovskites with compositions where X = 0-50%, whereas additional, lower intensity peaks appear when X = 70%. The three main α, β, and γ kinds of oxygen are placed at temperature regions of ca. 200-450 • C, 350-600 • C, and 600-800 • C, respectively; their reduction regions gradually shift to higher temperatures by Nanomaterials 2023, 13, 2271 9 of 26 increasing X until X = 50%, then, they shift again to lower temperatures when X = 70%, in accordance with Ponce et al. [78].
In accordance with the literature, peak α is attributed to adsorbed surface oxygen species, O ads ; peak β is attributed to the labile lattice oxygen, thus reflecting the Mn 4+ /Mn 3+ redox couple (Equation (4)) caused by the reduction of high-valence Mn 4+ to Mn 3+ ; and finally, peak γ is attributed to the labile lattice oxygen reflecting the Mn 3+ /Mn 2+ redox couple (Equation (5)) caused by the reduction of Mn 3+ to Mn 2+ [25,44,[78][79][80][81][82][83]. Equations (2) and (3) are written in accordance with the Kröger-Vink notation, as follows [84]: where the reference valence for manganese is considered to be +3. As we shall see below, LS X M perovskites are active during lean CH 4 combustion in the entire temperature region of ca. 250-800 • C, indicating that depending on the operation temperature, all these oxygen types could potentially contribute to the overall methane consumption rate.
Considering the combined XRD and H 2 -TPR findings, it may be concluded that the higher the X, the higher the M 4+ state content in pre-oxidized LS X M, and consequently, the higher the OSC of the material (Table 1). On the other hand, the tendency to reduce manganese (i.e., the lability of lattice oxygen, peaks β and γ) becomes progressively more difficult with increasing levels of X, up to 50%; then, for X = 70%, this is reversed (Figure 1). This can be understood if one considers that gradually increasing quantities of Mn 4+ are needed to compensate for the Sr 2+ additions; this finding is in accordance with the results reported in the literature [78], and it is reflected by the fact that the equilibrium constant of the reaction (4) is reported to decrease with increasing levels of X [85].
The addition of Ir to LS X M shifts the reducibility peaks of the obtained Ir/LS X M material to temperatures as low as ca. 150-400 • C, in which narrow temperature range encompassing all peaks (reflecting Ir 4+ → Ir 0 , Mn 4+ → Mn 3+ and Mn 3+ → Mn 2+ reductions) they substantially overlap ( Figure S4). This noble metal-induced enhancement of the support's reducibility is due to an increased spillover of hydrogen atoms from the noble metal particles to the reducible support, thus promoting the reducibility of the latter [74].
The additional, low-intensity peaks that appeared at temperatures of ca. 280, 560, and 750 • C on the LS 70 M H 2 -TPR spectrum presumably originate from the reduction of the additional oxide phases that appear to be present in the XRD pattern of this perovskite ( Figure S2). For example, manganate oxide phases can present with reduction peaks at low and intermediate temperature regions following successive reduction processes ( [86]. However, the low amount of these phases detected on LS 70 M only marginally affects the overall qualitative/quantitative reducibility behavior of the material, which is still mainly determined by the α, β, and γ peaks (Figure 1).

Light-Off/Light-Out Performance of LS X M and Ir/LS X M Catalysts
Following the experimental schedule described in Scheme 1 for catalytic performance data acquisition, the light-off (heating)/light-out (cooling) temperature cycles were conducted after different pre-treatment stages, involving fresh or aged (at 750 • C), pre-oxidized or pre-reduced, LS X M and Ir/LS X M catalysts, respectively. The results of the catalytic behavior, in the order they were obtained (Scheme 1), are discussed below (i.e., first those concerning fresh catalysts are discussed, followed by those concerning thermally aged catalysts, and so on).   It is apparent that the light-off (heating) and light-out (cooling) performances of the CH 4 conversion coincide to a reasonable extent. Therefore, no hysteresis phenomena were observed. However, the activity of the catalysts appears to be strongly dependent on the La substitution with Sr (X) in the perovskite, which can be seen more clearly with the variation in T 50 versus X (Figure 2c). This dependence, which is qualitatively similar for the two series of materials (LS X M and Ir/LS X M), is not monotonic, showing inverted volcanic behavior (with respect to activity), with the LS 50 M and Ir/LS 50 M pair appearing to be the less active. Furthermore, and rather unexpectedly, the iridium-loaded perovskites (Ir/LS X M) appear typically less active than their pristine perovskite counterparts (LS X M).

Pre-Reduced Fresh LS X M and Ir/LS X M Catalysts
The light-off (heating)/light-out (cooling) behaviors for the two sets of fresh catalysts, LS X M and Ir/LS X M, starting from their pre-reduced state, are depicted in Figure 3. After this pretreatment, the behavior of the catalysts appears more complex than their pre-oxidized counterparts, as it involved hysteresis phenomena (steady-state rate multiplicity) [30,[87][88][89][90]. Both kinds of hysteresis were obtained for the pre-reduced samples during heating/cooling cycles (i.e., normal (counterclockwise) or inverse (clockwise) hysteresis as commonly described in the literature [87], although the latter is clearer (more intense)). In particular, the pair (LS 00 M, Ir/LS 00 M), when X = 0%, follows normal hysteresis (with the activity possessing higher values upon cooling), whereas those with intermediate values, regarding the Sr substitution (X = 30 and 50%), obey inverse hysteresis. The hysteresis behavior of the pair (LS 70 M, Ir/LS 70 M), with the maximum (X = 70%) substitution of La with Sr, is complicated; the former catalyst follows normal hysteresis, and the latter follows inverse hysteresis, although, the amplitude of the hysteresis loop in both cases is limited. Interestingly, the hysteresis loops between the counterpart catalysts, LS X M and Ir/LS X M, are mirrored with respect to their qualitative characteristics, suggesting that hysteresis mainly originates from the perovskite compartment of the catalyst, rather than from Ir. The latter mainly affects the hysteresis amplitude.
Regarding the activity ranking of the pre-reduced catalysts, this can be followed best using their T 50 behavior, as depicted in Figure 3c. Similar to that observed for the pre-oxidized samples (Figure 2c), an inverted volcano-type behavior of the activity with increasing substitutions (X) of La with Sr is once again clear (Figure 3c). Thus, LS X M and Ir/LS X M, when X = 0%, outperformed those when X = 30%, to a greater extent than those when X = 50% (the least active). Then, catalysts with X = 70% appeared more active than those with intermediate X values, approaching the optimal activity of those when X = 0%. Moreover, during the heating part of the light-off/light-out cycle, Ir/LS X M catalysts appeared to exhibit a slightly superior performance compared with their LS X M counterparts, except in the case of the LS 50 M-Ir/LS 50 M pair ( Figure 3c); these behaviors are almost opposite to those recorded for the pre-oxidized samples (Figure 2c). The same is true for the catalysts where X = 0 and 30% (low Sr content), but not for cases where X = 50% and 70% (high Sr content) during the cooling part of the cycle.
When comparing the behaviors of pre-reduced and pre-oxidized LSxM ( Figure S5a) and Ir/LSXM ( Figure S5b) catalysts, a clear trend is revealed; in the vast majority of cases, the cooling part of the pre-reduced catalysts better approximates, or even coincides with, the behavior of their pre-oxidized counterparts. A typical example (for the Ir/LS 30 M catalyst) is shown in Figure 3d.

Light-Off/Light-Out Performance of LSXM and Ir/LSXM Catalysts Aged at 750 °C
Herein, the behavior of CH4 conversion as a function of temperature during heating/cooling cycles between 400 and 900 °C is presented for LSXM and Ir/LSXM catalysts that were previously subjected to oxidative thermal aging for 5 h, as described in Scheme 1. Both the pre-oxidized and pre-reduced states of the samples at the beginning of the cycle were investigated.
3.3.1. Pre-Oxidized Catalysts Aged at 750 °C, Figure 4 presents the CH4 conversion results obtained during the heating/cooling cycle for the pre-oxidized LSXM (Figure 4a) and the Ir/LSXM (Figure 4b) catalysts that were aged at 750°C, , whereas Figure 4c depicts the corresponding T50 behavior of the catalysts as a function of X. As for fresh catalysts (Figure 2), no hysteresis phenomena were observed during the cycles over the pre-oxidized, aged at 750°C samples. It was also observed that pure perovskites (LSXM) slightly outperformed their Ir-doped counterparts (Ir/LSXM), except for LS00M (Figure 4c).
Regarding the dependence of the activity on the Sr content of LSXM, the aged catalysts maintained the same behavior for their fresh counterparts (Figure 2). It is once again an inverted volcanic-type dependent activity competing against X, with LS50M and

Light-Off/Light-Out Performance of LS X M and Ir/LS X M Catalysts Aged at 750 • C
Herein, the behavior of CH 4 conversion as a function of temperature during heating/cooling cycles between 400 and 900 • C is presented for LS X M and Ir/LS X M catalysts that were previously subjected to oxidative thermal aging for 5 h, as described in Scheme 1. Both the pre-oxidized and pre-reduced states of the samples at the beginning of the cycle were investigated. Figure 4 presents the CH 4 conversion results obtained during the heating/cooling cycle for the pre-oxidized LS X M (Figure 4a) and the Ir/LS X M (Figure 4b) catalysts that were aged at 750 • C, whereas Figure 4c depicts the corresponding T 50 behavior of the catalysts as a function of X. As for fresh catalysts (Figure 2), no hysteresis phenomena were observed during the cycles over the pre-oxidized, aged at 750 • C samples. It was also observed that pure perovskites (LS X M) slightly outperformed their Ir-doped counterparts (Ir/LS X M), except for LS 00 M (Figure 4c).

Pre-Oxidized Catalysts Aged at 750 • C
Regarding the dependence of the activity on the Sr content of LS X M, the aged catalysts maintained the same behavior for their fresh counterparts (Figure 2). It is once again an inverted volcanic-type dependent activity competing against X, with LS 50 M and Ir/LS 50 M catalysts exhibiting inferior performances, with T 50 values approximately 250 • C higher than those for LS 00 M and Ir/LS 00 M (Figure 4c). Ir/LS50M catalysts exhibiting inferior performances, with T50 values approximately 250 °C higher than those for LS00M and Ir/LS00M (Figure 4c).

Pre-Reduced Catalysts Aged at 750 • C
The performance of the pre-reduced LS X M and Ir/LS X M catalysts that were aged at 750 • C, and their impact on the lean CH 4 oxidation reaction during heating/cooling cycles, are depicted in Figure 5a  The CH 4 conversion efficiency performance versus X of the pre-reduced catalysts aged at 750 • C, in terms of T 50 , is shown in Figure 5c. There are some variations in T 50 values between the heating and cooling parts of the cycle due to the hysteresis, which generally indicates more active catalysts in the heating part of the loop. Since the thermal cycle starts with pre-reduced catalysts, it is possible that in this part of the cycle, the preimposed metallic Ir 0 is partially oxidized by the oxidizing environment of the reactants (lean conditions), creating an IrO 2 /Ir 0 complex state. As demonstrated by Rui and coworkers [91], this offers favorable sites for the overall methane oxidation reaction, compared with metallic Ir 0 or stoichiometric IrO 2 . In the same vein as the results of Schick et al. [31], regarding the total oxidation of VOCs over Ir/SiO 2 catalysts, which showed that surface Ir 3+ species are associated with improved catalyst performance as well as size-activity relationship; regarding the latter, this is because as the size of the Ir nanoparticles decreases, the defective Ir 3+ active sites increase.
However, when the system achieves 100% CH 4 conversion at a high temperature, i.e., it operates in net oxidizing conditions, (due to the remained unconsumed O 2 , the Ir nanoparticles are fully oxidized into stoichiometric IrO 2 , therefore, they become less active in the reaction, and during its return to low temperatures (cooling) the system exhibits lower activity.
Nevertheless, the effect of substituting La with Sr (X) remains the main cause of strong changes in terms of the efficiency of the catalysts. Changes in the order of~60 • C max. were obtained between the heating and cooling sections of the hysteresis loop; these are insignificant compared with changes of 250 • C, caused by the 50% replacement of La with Sr.

Thermal Aging and Time-on-Stream Stability of Catalysts
In order to evaluate the thermal stability of the materials after prolonged exposure to thermal aging at high temperatures under oxidizing conditions (5 h at 750 • C in a 20% O 2 /He flow), Figure 6 was constructed, which presents the efficiency (in terms of T 50  It is evident from Figure 6 that aging causes the degradation of the catalytic efficiency of the studied materials, but this is rather insignificant, as in most cases, this results in only slight increases in T 50 (ca. +20 • C). Furthermore, in the case of catalysts with X = 0 and 70%, the activity of the catalysts, either of unloaded or Ir-loaded perovskites, remains unaffected after aging. The thermal aging stability of perovskites under oxidizing conditions is well established, and its confirmation it was not surprising to obtain this result for the pristine LSxM (Figure 6a-c). Moreover, the strong susceptibility and tendency to aggregate, regarding the supported iridium nanoparticles under oxidative thermal aging conditions, is also known [37,38], and thus, it was the focus of the stability experiments. However, the results of Figure 6d-f are similar to those of the pristine perovskites; the Ir/LS X M catalysts exhibited good stability, and an explanation for this is given below.
Regarding the time-on-stream stability (TOS) of our catalysts, experiments were performed for a period of 12 h (Figure 7) under constant temperatures that corresponded with the T 50 exhibited by each of them. The less active catalysts, LS 50 M and Ir/LS 50 M, with high T 50 values (~900 • C), were operated at temperatures of 850 • C; at such temperatures, they have an initial CH 4 conversion rate of about 20-25% (Figure 7). All catalysts showed very good stability, since only a slight degradation, in the order of 5%, was recorded after 12 h of operation. It is worth noting that the catalyst pair, LS 00 M and Ir/LS 00 M, was the most active compared with all others investigated, and it also offered the best TOS stability, with zero degradation in terms of their efficiency, over the 12 h period.

Thermal Aging and Time-on-Stream Stability of Catalysts
In order to evaluate the thermal stability of the materials after prolonged exposure to thermal aging at high temperatures under oxidizing conditions (5 h at 750 °C in a 20% O2/He flow), Figure 6 was constructed, which presents the efficiency (in terms of T50) of fresh and aged (at 750 °C) LSXM (Figure 6a-c) and Ir/LSXM (Figure 6d-f) catalyst series.  high T50 values (~900 °C), were operated at temperatures of 850 °C; at such temperatures, they have an initial CH4 conversion rate of about 20-25% (Figure 7). All catalysts showed very good stability, since only a slight degradation, in the order of 5%, was recorded after 12 h of operation. It is worth noting that the catalyst pair, LS00M and Ir/LS00M, was the most active compared with all others investigated, and it also offered the best TOS stability, with zero degradation in terms of their efficiency, over the 12 h period.

Main Observations and Material Properties-Catalytic Efficiency Correlations
The main observations of the above comparative studies of catalytic behavior (efficiency and stability) of the two series of LS X M and Ir/LS X M catalysts for the complete CH 4 oxidation reaction under lean conditions, can be summarized as follows: (i) The most important determining factor of the materials, for all cases (unloaded or Ir-loaded LS X M, pre-oxidized or pre-reduced, and fresh or aged materials), in terms of efficiency, was the composition of the LS X M perovskite, specifically, the amount of La that was substituted with Sr (X = 0, 30, 50, and 70%). This key factor typically causes shifts in T 50 of up to ca. 300 • C, whereas other parameters appeared capable of shifts in T 50 that were one order of magnitude lower (ca.  (Figures 2c-5c and 6). (iii) The addition of Ir nanoparticles onto the LS X M surface did not perform as expected.
It had a negative effect on the efficiency of pre-oxidized catalysts (Ir/LS X M show T 50 values that were~30 • C higher than those of their non-loaded LS X M counterparts) (Figures 2c and 4c), whereas the pre-reduced catalysts exhibited a small positive effect at high temperatures, but it was more noticeable for lower temperatures (Figures 3c  and 5c). (iv) Hysteresis phenomena during heating/cooling cycles appeared only in the case of pre-reduced catalysts for both LS X M and Ir/LS X M series (Figures 3 and 5), whereas for pre-oxidized materials, the light-off and light-out curves faithfully coincided with each other (Figures 2 and 4).
(v) The hysteresis loops are qualitatively similar for the corresponding LS X M and Ir/LS X M catalysts, which indicates that the phenomenon is mainly determined by LS X M and its properties, rather than Ir (Figures 3 and 5). (vi) The oxidative thermal aging of the materials caused marginal decreases in their efficiency (i.e., less than~40 • C shifts of T 50 towards higher temperatures). However, for the most active catalysts (with X = 0 and 70%), no activity deterioration was recorded ( Figure 6). (vii) The time-on-stream stability of the materials was generally good, as the efficiency (methane conversion) only declined by 5-10%; this was observed after 12 h of operation. Notably, no degradation in the catalytic efficiency of the optimal LS 00 M (LaMnO 3 ) and Ir/LS 00 M (Ir/LaMnO 3 ) catalysts was recorded (Figure 7).
After a close comparison of the LS X M characteristics (and how these change in accordance with the extent (X) to which La was substituted with Sr) with the catalytic results, we can readily conclude that the main factors affecting the activity of the perovskites during the lean CH 4 combustion are the total surface area (S BET ) and the reducibility of the perovskite. Indeed, as shown in Figure 8a and b, respectively, the efficiency of the LS X M (in terms of T 50 ) faithfully mimics the changes in both the total surface area (S BET ) and reducibility (expressed in terms of the peak temperature of Mn 4+ → Mn 3+ reduction in the TPR profile of Figure 1) when the level of substituted Sr (X) in the perovskite increases.
Studies of the literature, regarding lean CH 4 combustion over perovskite-type materials, typically describe the reaction mechanism via the Eley-Rideal (ER), Mars-van Krevelen (MVK), and/or both (two-term) kinetic models [63,67,79]. The ER model involves the direct reaction of gas phase methane with adsorbed oxygen species (O ads ) on the perovskite surface. The MVK model implicates the reaction of gas phase CH 4 with surface lattice oxygen (Equation (6)), the latter being continuously replenished by the dissociative chemisorption of O 2 on surface oxygen vacancies (Equation (7)), as follows: Performing Density Functional Theory (DFT) calculations for CH 4 combustion on LSM perovskites, Wang et al. [92] also indicated that CH 4 adsorption and activation (CH 4 * → CH 3 * + H*) occurs on Mn sites. They argued that this activation, revealed to be more facile on the (001) rather than (110) crystallographic plane, can cause further enhancements to catalytic methane oxidation after a reaction concerning such methane-derived species with neighbor lattice oxygen atoms takes place.
It is therefore obvious from the above considerations that perovskite activity, regarding the reaction under consideration, will be directly proportional to the population of labile oxygen species on the perovskite surface, and thus, to the perovskite surface area itself. At the same time, the degree of lability of these lattice oxygen species would act synergistically upon the enhancement of the activity.
Observations (i) and (ii) that are related to the entire catalytic efficiency data, either obtained on fresh or aged, or on pre-oxidized or pre-reduced, catalysts, they exhibit an activity order (LS 00 M~LS 70 M > LS 30 M > LS 50 M) that is fully consistent with the above. The higher the perovskite surface area and lattice oxygen lability, the higher its efficiency in terms of lean CH 4 combustion (Figure 8c,d). As we have seen here, increasing the amount of substituted Sr in LS X M, up to X = 50%, gradually degrades both of these activitydetermining parameters; for X = 70%, however, the formation of other simple oxide phases (e.g., Mn 2 O 3 , see XRD results in Figure S2) may be the cause of the recovery of high values in said properties, and therefore, the return of efficiency to high values. activity order (LS00M ~ LS70M > LS30M > LS50M) that is fully consistent with the above. The higher the perovskite surface area and lattice oxygen lability, the higher its efficiency in terms of lean CH4 combustion (Figure 8c,d). As we have seen here, increasing the amount of substituted Sr in LSXM, up to X = 50%, gradually degrades both of these activity-determining parameters; for X = 70%, however, the formation of other simple oxide phases (e.g., Mn2O3, see XRD results in Figure S2) may be the cause of the recovery of high values in said properties, and therefore, the return of efficiency to high values.  The unexpected inhibitory effect of adding Ir to the surface of LS X M perovskites, regarding their catalytic efficiency (observation iii), is still uncertain. As shown in Table 1, the Ir/LS X M catalysts typically have a lower surface area compared with their LS X M counterparts due to their partially blocked pores, as explained in Section 3.1. Therefore, part of the surface of the perovskite support, which is intrinsically active in the reaction, is rendered inaccessible to the reactants mixture. The catalyst thus loses active centers, which, in the case of pre-oxidized samples, are not replenished by the presence of, relatively inactive to the reaction, IrO 2 phase. In the case of the pre-reduced Ir/LS X M catalysts, this undesirable effect of the iridium species is partially compensated with the creation of an active for the reaction IrO 2 /Ir 0 phase (partially oxidized iridium) [31,91]. This occurs after the exposure of the Ir 0 species of the pre-reduced catalysts to the excess O 2 conditions. However, as our results show, this favorable partially oxidized surface of the iridium particles (IrO 2 /Ir 0 ) created on the pre-reduced catalysts is not expected to remain in high temperatures and under excess O 2 reaction conditions. It transforms to the less active IrO 2 which moderates positive effects (Figures 3 and 5).
The consistent interpretations of observations (iv) and (v), which are related to inverse hysteresis phenomena (Figures 3 and 5), and are recorded in this study, are as follows: Why is hysteresis observed only on the pre-reduced, and not the pre-oxidized, catalysts, and why is it of the inverse type (i.e., clockwise, the heating part outperforms its cooling part)? The light-off (heating part) behavior of the pre-reduced catalysts (Figures 3  and 5) is very similar to the behavior first observed by Arai et al. [67], who studied CH 4 combustion on La 0.8 Sr 0.2 MnO 3 and La 0.6 Sr 0.4 MnO 3 perovskites using a reaction mixture of 2% v/v CH 4 in air. It exhibits a characteristic "hump" in the low-temperature region, then, it changed curvature at higher temperatures, as also found herein (Figures 3 and 5). We agree with the explanation given by the authors, as follows. The initial high rate of CH 4 consumption is the result of the contribution to the overall rate of adsorbed oxygen species (O ads ) on the perovskite surface which is then limited by increasing temperatures (at high temperatures the O ads desorb rapidly); however, at high temperatures the participation of lattice oxygen (O × O ) in methane oxidation becomes dominant [67]. Regarding the behavior of the cooling (light-out) part of the hysteresis loops (Figures 3  and 5), we can give the following rational explanation. When approaching~100% methane conversion at the upper-temperature limit, the catalyst experiences pure oxidative conditions (~100% CH 4 conversion, excess O 2 ). It is therefore oxidized under such conditions, and by lowering the temperature, it follows a path of lower activity, similar to the results obtained in pre-oxidized samples (Figures 2 and 4). This explains why the cooling part of the hysteresis loops was always similar to the light-off curves of the pre-oxidized catalysts, in addition to the "inverse" character of the hysteresis.
In accordance with the above, hysteresis mainly occurs due to the favorable contribution of O ads species to the total methane consumption rate. However, the reason why this was found to only work for pre-reduced samples, but not pre-oxidized samples, can be interpreted as follows. The pre-reduction process of LSxM materials is expected to enhance the population of oxygen vacancies on their surface due to a partial Mn 4+ → Mn 3+ reduction. When such a pre-treated LS X M perovskite experiences in methane oxidation environments, the gas phase dioxygen interacts with oxygen vacancies, providing two atomic oxygens, one of which is bound by the vacancy that becomes the lattice oxygen, and the other can diffuse to the perovskite surface in the form of adsorbed O ads . Thus, a mechanism of continuous replacement concerning both lattice and adsorbed oxygen species is at work during the reaction on pre-reduced materials. Conversely, the pre-oxidized perovskites, due to the lack of oxygen vacancies on their surface to activate the dissociative chemisorption of gaseous dioxygen in this temperature range, have a limited amount of O ads that impose low reaction rates. Thus, for per-oxidized samples, both the heating and cooling parts of the light-off experiments coincide since the samples work for both paths in their oxidized state.
The presence of Ir in the catalyst formulations (Ir/LS X M) appears to have some noticeable effect on the hysteresis phenomena, especially on loop amplitude, regarding CH 4 conversion. It appears strongly enlarged ( Figure 5b) compared with that recorded in the corresponding LS X M catalysts (Figure 5a). A reasonable explanation is that the partially oxidized IrO 2 /Ir 0 sites (their creation has been described above), which are favorable for the dissociative adsorption of gaseous dioxygen, further facilitate this necessary mechanistic step of the reaction. The as-derived oxygen atoms can then directly react with the methane, and/or spillover to the perovskite surface, thus replenishing the lattice oxygen consumed by the reaction, enhancing methane combustion.
Regarding observation (vi), which concerns the good stability of our materials in spite of the fact that they were subjected to arduous conditions, even those containing Ir (Ir/LS X M), is an impressive finding considering that Ir is a very sensitive catalyst with regard to thermal aging; it has a high aggregation tendency, even at temperatures as low as 450 • C [13,14,[37][38][39][40][41]. The present results once again confirm our recent experimental findings, theory, and developed model, demonstrating that supports possess a sufficient population of bulk and surface O-defects (oxygen vacancies), and a high mobility of lattice O 2− ions effectively act against the thermal sintering of catalyst nanoparticles dispersed on their surface [40,41]. The perovskites used herein as supports for Ir nanoparticles meet these requirements.
Observation (vii), on the other hand, concerning the good time-on-stream stability of our materials, most likely reflects the absence of the accumulative carbon deposition phenomena originating from the methane cracking reaction, the occurrence of which is not thermodynamically excluded in the temperature range used for the reaction in this study.
An additional aspect worth noting in the present findings concerns the fact that, in the literature, it is known that at high temperatures (ca. > 850 • C, reached in the present study) the contribution of the gas phase to the total methane oxidation can be appreciable under certain conditions and should not be ignored. Matzaras and co-workers, in a Cartesian plot of SV (surface-to-volume ratio) versus residence time, delineated the regimes concerning the significant involvement of gas-phase chemistry for given pressures and temperatures [93]. Considering the conditions in which the present experiments were performed (SV > 200,000 and residence time~0.017 s at a pressure of 1 bar), it seems that the catalytic system operates under conditions that starkly differ with those wherein the contribution of the gas-phase reaction could be noticeable. This view is further strengthened by two additional facts, as follows: first, no slope change tendency of the light-off curves at high temperatures is observed in Figures 2-5; second, nor was CO detected in the reaction products (the contribution of the gas phase goes through the reaction sequence: CH 4 → CO gas-phase route followed by the catalytic oxidation of the produced gaseous CO to CO 2 ).
Finally, taking into account the small particle size of the catalytic powdered samples (180-250 µm), and the high GHSV used (ca. 156,000 h −1 ), mass and heat transfer limitations, if any, should be negligible.

Conclusions
The complete oxidation of the methane reaction was investigated in detail on a series of LS X M (X = % substitution of La with Sr = 0, 30, 50, and 70%) perovskites, and on their 2 wt% Ir-loaded counterpart catalysts.
The key parameter affecting efficiency for both series concerned the degree (X) of substitutional Sr, rather than the addition of Ir; the latter caused a slight inhibition of activity, rather than promotion, when in its oxidation state (IrO 2 ), and it notably enhanced activity when in a partially oxidized state (IrO 2 /Ir 0 ).
The main factors through which parameter X affects the activity were the changes it induced in the materials' total surface area and on their reducibility that act synergistically. The materials' efficiency followed an inverted volcanic behavior pattern, as a function of X, with the most efficient catalysts being the LS 00 M-Ir/LS 00 M pair, and the least efficient being the LS 50 M-Ir/LS 50 M pair; this strikingly reflects the effect of X on the total surface area and the reducibility of the materials. Large differences in T 50 (ca. 300 • C) were recorded between the most and least active catalysts.
The inhibitory effect on efficiency, caused by the addition of Ir to the perovskite, was attributed to partial pore blocking, and consequently, to a reduction in the number of active sites, in combination (for pre-oxidized catalysts) with the low activity of IrO 2 particles. For pre-reduced catalysts, the creation under reaction conditions of a reactive partially oxidized IrO 2 /Ir 0 phase enhanced activity, especially at low temperatures.
Inverse hysteresis phenomena recorded on pre-reduced, but not pre-oxidized, LSxM and Ir/LS X M catalysts during light-off (heating)/light-out (cooling) temperature cycles and overall reaction kinetics were consistently interpreted by assuming that CH 4 oxidation was performed in parallel with O ads species (Eley-Rideal model) and lattice oxygens (Mars-van Krevelen model) (i.e., a two-term kinetic model).
Both the LS X M and Ir/LS X M series of materials were found to be very stable in both oxidative thermal aging tests at 750 • C for 5 h, and the 12 h time-on-stream stability tests under the reaction's conditions.
The great stability of perovskites at high temperatures for the total oxidation of CH 4 , the convenience they provide for effectively tailoring their performance via the partial substitution of the A or B sites with other cations, and the efficient CH 4 conversion they achieve, even without noble metal loading, make them promising materials that are worthy for future studies concerning demanding applications such as exhaust gas treatment in NG-driven vehicles or gas turbines (GTs).

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13152271/s1: Figure S1. Schematic configuration of the continuous flow experimental apparatus equipped with on-line Gas Chromatography; Figure S2. XRD patterns of LS X M perovskites (La 1−x Sr x MnO 3 ; X = 0, 30, 50 and 70% substitution of La by Sr) and the corresponding 2 wt% Ir/LS X M catalysts; Figure S3. Magnification of the XRD patterns of LS X M perovskites at the region 2θ = 32-34 • , where the main peak of the perovskite phase appears; Figure S4. H 2 consumption versus temperature (H 2 -TPR profiles) of Ir/LS X M catalysts. Figure S5. Dependence of T 50 on X (% substitution of La by Sr in the perovskite composition) for pre-reduced and pre-oxidized LS X M catalysts (a), and pre-reduced and pre-oxidized Ir/LS X M catalysts (b); Table S1. Temperature for 50% CH 4 conversion (T 50 ) of pre-oxidized and pre-reduced, fresh LS X M and Ir/LS X M catalysts; Table S2. Temperature for 50% CH 4 conversion (T 50 ) of pre-reduced and pre-oxidized LSxM and Ir/LSxM catalysts that were aged at 750 • C.

Conflicts of Interest:
The authors declare no conflict of interest.